1. Field of the Invention
The present invention relates to a method and apparatus of removing nitrogen, and more particularly, to a method and apparatus of removing nitrogen, in which ammonium nitrogen is removed from wastewater containing a BOD component and ammonium nitrogen by anaerobic ammonium oxidation.
2. Description of the Related Art
Nitrogen components contained in sewage and industrial effluent cause eutrophication in lakes and lower the level of dissolved oxygen in rivers. For these reasons, they must be removed. Examples of nitrogen components contained in the sewage and industrial effluent primarily include ammonium nitrogen, nitrite nitrogen, nitrate nitrogen, and organic nitrogen.
Conventionally, when the concentration of nitrogen in such wastewater is low, an ion exchange method and oxidation with chloride or ozone are used to remove nitrogen. When the concentration of nitrogen is medium or high, biological treatment is employed and usually operated in the following conditions.
In the biological treatment, nitrification/denitrification treatment is performed by both aerobic nitrification and anaerobic denitrification. In the aerobic nitrification, ammonium nitrogen and nitrite nitrogen are oxidized by ammonium oxidation bacteria (Nitrosomonas, Nitrosococcus, Nitrosospira, Nitrosolobus, etc.) and nitrite oxidation bacteria (Nitrobactor, Nitrospina, Nitrococcus, Nitrospira etc.). Whereas, in the anaerobic denitrification, denitrification is performed by heterotrophic bacteria (Pseudomonas denitrificans etc.).
Furthermore, a nitrification vessel for aerobic nitrification is operated under a load of 0.2 to 0.3 kg-N/m3/day, whereas a denitrification vessel for anaerobic denitrification is operated under a load of 0.2 to 0.4 kg-N/M3 per day. To treat nitrogen contained in a total concentration of 30 to 40 mg/L in sewage, the sewage must be retained in a nitrification vessel for 6 to 8 hours and in a denitrification vessel for 5 to 8 hours. To attain these treatments, a large treatment vessel is required. Furthermore, the nitrification vessel and denitrification vessel for use in treating industrial effluent containing only inorganic components are designed to treat the same loads as above. However, this case requires an organic material for denitrification, so that methanol is usually added in a concentration three or four times as large as a nitrogen concentration. Therefore, this case has a problem in that not only initial cost but also large running cost is required.
However, recently, anaerobic ammonium oxidation which is a method of removing nitrogen from wastewater has been remarkable as effective and economical nitrogen removal method (for example, Japanese Patent Application Laid Open No. 2001-37467). In the anaerobic ammonium oxidation, ammonium and nitrite are simultaneously removed by anaerobic ammonium oxidizing bacteria in accordance with the following reaction scheme:1.0NH4+1.32NO2+0.066HCO3+0.13H+→1.02N2+0.26NO3+0.066CH2O0.5N0.15+2.03H2O  (Formula 1)
This method has the following advantages. Since ammonium is used as a hydrogen donor, the amount of methanol to be used for denitrification can be significantly reduced and the amount of excess sludge can be decreased.